Inspection of lithographic mask blanks for defects

Abstract

A visible light method for detecting sub-100 nm size defects on mask blanks used for lithography. By using optical heterodyne techniques, detection of the scattered light can be significantly enhanced as compared to standard intensity detection methods. The invention is useful in the inspection of super-polished surfaces for isolated surface defects or particulate contamination and in the inspection of lithographic mask or reticle blanks for surface defects or bulk defects or for surface particulate contamination.

What is claimed is:  
     
       1. A method for enhancing a detected signal from light scattered from a particle on an object, comprising: 
       determining an approximate first angular distribution of particle scatter signal produced by a first portion of a laser beam scattered from a particle, wherein said laser beam has an angle of incidence with respect to said object;  
       determining an approximate second angular distribution of object scatter noise produced by said first portion of said laser beam scattered from said object;  
       calculating a ratio of said approximate first angular distribution to said approximate second angular distribution to produce an approximate third angular distribution of particle scatter signal to object scatter noise;  
       placing a detector at a position chosen from said approximate third angular distribution;  
       directing said first portion of said laser beam at said angle of incidence to said particle to produce scattered light;  
       frequency shifting at a frequency difference a second portion of said laser beam with respect to said first portion of said laser beam to produce a frequency shifted laser beam;  
       directing said frequency shifted laser beam at said object to produce specularly reflected light directed at said detector; and  
       detecting optical heterodyning, at said frequency difference, between a portion of said scattered light and a portion of said specularly reflected light to produce enhanced scattered light signal from said detector.  
     
     
       2. The method of claim  1 , wherein said object comprises a lithographic mask blank. 
     
     
       3. The method of claim  1 , wherein said particle has a size within a range of 18 nm to 180 nm. 
     
     
       4. The method of claim  1 , wherein the step of determining an approximate first angular distribution comprises utilizing a library. 
     
     
       5. The method of claim  4 , wherein said library comprises a database. 
     
     
       6. The method of claim  1 , wherein the step of determining the approximate first angular distribution comprises calculating said first angular distribution. 
     
     
       7. The method of claim  6 , wherein the step of calculating said first angular distribution comprises: 
       modeling the physical geometry and material properties of said particle by placing dipoles representing said particle in a lattice configuration;  
       calculating the dipole moment distribution within said particle to determine the response of each said dipole to said first portion of said laser beam; and  
       calculating said first angular distribution from said dipole moment distribution.  
     
     
       8. The method of claim  6 , wherein the step of calculating said first angular distribution comprises utilizing an electromagnetic simulation code. 
     
     
       9. The method of claim  8 , wherein said electromagnetic simulation code comprises the TSAR code. 
     
     
       10. The method of claim  8 , wherein said electromagnetic simulation code comprises the DDSURF code. 
     
     
       11. The method of claim  8 , wherein said electromagnetic simulation code comprises a code selected from a group consisting of TSAR, EMFLEX, EMINENCE, MAFIA and DDSURF. 
     
     
       12. The method of claim  1 , wherein the step of determining an approximate second angular distribution comprises utilizing a library. 
     
     
       13. The method of claim  12 , wherein said library comprises a database. 
     
     
       14. The method of claim  1 , wherein the step of determining the approximate second angular distribution comprises calculating said second angular distribution. 
     
     
       15. The method of claim  14 , wherein the step of calculating said second angular distribution comprises: 
       modeling the physical geometry and material properties of said particle by placing dipoles representing said particle in a lattice configuration;  
       calculating the dipole moment distribution within said particle to determine the response of each said dipole to said first portion of said laser beam; and  
       calculating said second angular distribution from said dipole moment distribution.  
     
     
       16. The method of claim  14 , wherein the step of calculating said second angular distribution comprises utilizing an electromagnetic simulation code. 
     
     
       17. The method of claim  16 , wherein said electromagnetic simulation code comprises the TSAR code. 
     
     
       18. The method of claim  16 , wherein said electromagnetic simulation code comprises the DDSURF code. 
     
     
       19. The method of claim  16 , wherein said electromagnetic simulation code comprises a code selected from a group consisting of TSAR, EMFLEX, EMINENCE, MAFIA and DDSURF. 
     
     
       20. The method of claim  1 , wherein said position chosen from said approximate third angular distribution comprises a position of optimal particle scatter signal to object scatter noise. 
     
     
       21. The method of claim  1 , wherein said position chosen from said approximate third angular distribution comprises a position of maximum particle scatter signal to object scatter noise. 
     
     
       22. The method of claim  1 , wherein the step of frequency shifting is carried out with an acousto-optic frequency shifter. 
     
     
       23. The method of claim  1 , wherein said frequency difference is within a range of 10 MHz to 1000 MHz. 
     
     
       24. The method of claim  1 , wherein said angle of incidence of said first portion of said laser beam comprises a plurality of angles of incidence. 
     
     
       25. The method of claim  1 , wherein said first portion of said laser beam comprises S polarization. 
     
     
       26. The method of claim  1 , wherein said first portion of said laser beam comprises P polarization. 
     
     
       27. A method for enhancing a detected signal from light scattered from a particle on an object, comprising: 
       calculating an approximate first angular distribution of particle scatter signal produced by a first portion of a laser beam scattered from a particle, wherein said laser beam has an angle of incidence with respect to said object;  
       calculating an approximate second angular distribution of object scatter noise produced by said first portion of said laser beam scattered from said object;  
       calculating a ratio of said approximate first angular distribution to said approximate second angular distribution to produce an approximate third angular distribution of particle scatter signal to object scatter noise;  
       placing a detector at a position chosen from said approximate third angular distribution;  
       directing said first portion of said laser beam at said angle of incidence to said particle to produce scattered light;  
       frequency shifting at a frequency difference a second portion of said laser beam with respect to said first portion of said laser beam to produce a frequency shifted laser beam;  
       directing said frequency shifted laser beam at said object to produce specularly reflected light directed at said detector; and  
       detecting optical heterodyning, at said frequency difference, between a portion of said scattered light and a portion of said specularly reflected light to produce enhanced scattered light signal from said detector.  
     
     
       28. A method for enhancing a detected signal from light scattered from a particle on an object, comprising: 
       determining a first angular distribution of particle scatter signal produced by a first portion of a laser beam scattered from a particle, wherein said laser beam has an angle of incidence with respect to said object;  
       determining a second angular distribution of object scatter noise produced by said first portion of said laser beam scattered from said object;  
       calculating a ratio of said first angular distribution to said second angular distribution to produce a third angular distribution of particle scatter signal to object scatter noise;  
       placing a detector at a position chosen from said approximate third angular distribution;  
       directing said first portion of said laser beam at said angle of incidence to said particle to produce scattered light;  
       frequency shifting at a frequency difference a second portion of said laser beam with respect to said first portion of said laser beam to produce a frequency shifted laser beam;  
       directing said frequency shifted laser beam at said object to produce specularly reflected light directed at said detector; and  
       detecting optical heterodyning, at said frequency difference, between a portion of said scattered light and a portion of said specularly reflected light to produce enhanced scattered light signal from said detector.  
     
     
       29. An apparatus for enhancing a detected signal from light scattered from a particle on an object, comprising: 
       means for calculating an approximate first angular distribution of particle scatter signal produced by a first portion of a laser beam scattered from a particle, wherein said laser beam has an angle of incidence with respect to said object;  
       means for calculating an approximate second angular distribution of object scatter noise produced by said first portion of said laser beam scattered from said object;  
       means for calculating a ratio of said approximate first angular distribution to said approximate second angular distribution to produce an approximate third angular distribution of particle scatter signal to object scatter noise;  
       a detector placed at a position chosen from said approximate third angular distribution;  
       means for directing said first portion of said laser beam at said angle of incidence to said particle to produce scattered light;  
       means for frequency shifting at a frequency difference a second portion of said laser beam with respect to said first portion of said laser beam to produce a frequency shifted laser beam; and  
       means for directing said frequency shifted laser beam at said object to produce specularly reflected light directed at said detector,  
       wherein said detector detects optical heterodyning, at said frequency difference, between a portion of said scattered light and a portion of said specularly reflected light to produce enhanced scattered light signal from said detector.